Many computer display systems in use today have both an all points addressable (APA) display mode and an alpha-numeric, or character, display mode. The APA display modes are increasingly important as they allow text, graphics and image data to be displayed. Character display modes (i.e. using fixed-size character boxes) while becoming less important, have advantages over APA modes in certain circumstances (e.g. for operating system messages) because they intrinsically have less demand for storage. Added to this character display modes remain necessary for reasons of compatibility with the large number of alpha-numeric applications already existing.
As APA display modes are currently seen as the most effective way of managing the display of computer generated information, a lot of development effort has been put into finding ways to improve the performance of these modes. With this in mind, it has been suggested that dual-ported display memory (in particular dual-ported video memory which is otherwise known as VRAM) should be used for the storage of data for display. A VRAM is a particular form of dynamic RAM (or DRAM) which, in addition to the usual DRAM random access mode, has a serial access mode in which data can be output sequentially at high speed in, for example an eight bit wide data stream. This fast serial access to data stored in a VRAM means that high video rate monitors can be supported. However the use of this technology poses a problem when a display system also has to provide a character mode, as the VRAM can only be accessed rapidly if the data stored in the memory is accessed sequentially. In a character mode, although the accessing of the character code and attribute information is sequential, the accessing of the font memory is not, and thus the font cannot be usefully stored in the VRAM. This problem is compounded in that prior display adapter standards such as the IBM Extended Graphics Array (EGA) and the IBM Video Graphics Array (e.g. VGA) which were based on DRAM technology, allowed a large number of fonts to be stored in their display memory, of which only a limited number could be displayed on a display device at any one time.
In a prior graphics standard (the IBM MCGA), a small static store, separate from the display memory was used for the storage of character fonts. However, only two character fonts could be displayed (both of these being held in the static RAM) with the result that MCGA adapters are incompatible with the EGA and VGA standards which require that more fonts can be dealt with.
European patent application EP-A-284,904 relates to a display system with a symbol font memory in which a selection of symbol fonts are stored in the system memory of a workstation and only those portions of a symbol font which are currently needed for display are transferred to the display memory of a display system. In this way part of the APA display memory is configured as a cache. This prior display system addresses the system overhead incurred in updating the display memory from the system memory of the workstation, but does not address the problem the instant invention seeks to solve, namely the efficient support of character modes in a display system comprising a dual-ported display memory. Indeed, the invention to which EP-A-284,904 relates is illustrated by two examples, both of which are based on prior display adapter standards which use DRAM technology; namely the Colour Graphics Adapter (CGA) and the Extended Graphics Array (EGA). It should be noted that that the term "character font" as used herein is intended to be synonymous with the term "symbol font" used in EP-A-284,904.